High resolution mapping of soil geochemistry at a regional scale (Wallonia, South Belgium)

In many countries, government agencies rely on soil maps of trace metal background concentrations to implement soil protection policies. In Wallonia (South Belgium), regional regulation requires extremely detailed background concentration maps. The main obstacle to obtain these maps are the large variations in local background concentrations over short distances, mainly due to the high diversity of soil parent materials, and the atmospheric deposition of pollutants around former base metal smelters. Here we outline the developed methodology and we present an example of the achieved geochemical maps at land parcel scale. The methodology established for the geochemical mapping of soils involves three steps. In the first step, we divided the Wallonia (17,000 km²) in pedogeochemical units of soil based on (1) the soil map of the Wallonia (1:20,000), (2) geological maps (1:40,000 and 1:25,000), and (3) literature data and surveys results about soils and geology of Wallonia. In the second step, we gathered about 17,500 georeferenced soil geochemical data from environmental surveys (that we carried out) or from regulatory analyses (required for agricultural use of sludge from sewage plants). On the last step, we drew the maps by using geostatistical model based on the map of the pedogeochemical soil units and the georeferenced trace metals analyses. The resulting maps show that Wallonia has higher Ni background concentration in soil than average values in Europe (according to the values of Geochemical Atlas of Europe), principally because of elevated geogenic background levels. Also, higher Zn and Cd values are found, principally due to atmospheric deposition of pollutants originating from former base metal smelters. Thanks to their resolution, the maps make it possible to predict, for instance, the risk of exceeding threshold metal concentrations in any particular land parcel in Wallonia